1. Field of the Invention
The present invention relates to electrical communications systems and, in particular, to a differential phase shift keying modulator that modulates a carrier waveform utilizing pulse density modulation (PDM).
2. Discussion of the Prior Art
The basic function of a communications system is to transmit information from a source to a destination as fast and as accurately as possible. The source and the destination are physically separated from one another and are connected by a communications channel.
There are two types of information sources: analog information sources and discrete information sources. Analog sources, such as a microphone, produce a continuous signal. Discrete sources, such as a digital computer, generate a signal consisting of a sequence of pulses. Analog signals can be converted to discrete signals by utilizing sampling and quantizing techniques.
Communications channels which are designed to handle voice transmissions (i.e., the telephone network) have characteristics which make it difficult for them to transmit digital signals. To permit the transmission of digital bit streams over a voice channel, it is necessary to utilize the digital data pulses to modulate a carrier waveform having a frequency which is compatible with the voice channel.
The equipment which performs the required modulation is generally referred to as a "modem". The term "modem" is an acronym for MOdulator-DEModulator, since the equipment typically includes the capability not only to modulate transmitted signals but also to demodulate received signals.
Generally speaking, as stated above, a modulator receives a serial digital data bit stream from an information source and converts the bit stream to a waveform suitable for transmission over the communication channel. In addition to matching the frequency spectrum of the transmitted signal with the characteristics of the communication channel, the modulator also minimizes the effect of signal distortion caused by the non-ideal nature of the communications channel.
There are three basic modulation types: (1) amplitude-shift keying (ASK), (2) frequency-shift keying (FSK), and (3) phase-shift keying (PSK).
Data transmission systems which operate at lower data rates, i.e. 1200 baud or less, typically utilize FSK modulation. In FSK modulation, the two binary states are represented by two different frequencies and are detected by using two frequency tuned sections, one tuned to each of the two bit frequencies. The demodulated signals are then integrated over the duration of a bit and a binary decision is based on the result.
For systems that use higher data rates, various forms of PSK modulation are generally utilized.
A 2-phase PSK modulator uses one phase of the carrier frequency for one binary state and a second phase for the other binary state. The two phases are 180.degree. apart and are detected by a synchronous detector using a reference signal at the receiver which is of known phase with respect to the incoming signal.
In a 4-phase PSK modulator, the carrier waveform is switched 0.degree., 90.degree., 180.degree. or 270.degree. relative to the reference to represent a dibit data signal.
In a variation of the basic PSK modulation technique, differential phase-shift keying (DPSK), the digital pulse information is contained in the phase change between adjacent pulse intervals. For example, in the commonly-used .+-.45.degree., .+-.135.degree. 4-phase DPSK system, a phase change from one interval to the next of +45.degree. encodes the binary code 11, +135.degree. encodes 01, -135.degree. encodes 00, and -45.degree. encodes 10.
The advantage of DPSK over PSK is that there is no need to transmit a phase reference. All that is required at the receiver end is a short-term memory device to store the phase for one pulse interval.
Conventional analog phase modulators require complicated base-band filtering circuits to shape the in-band frequency spectrum. In addition, expensive carrier multipliers, summers and in-phase/quadrature phase splitter networks are usually involved in designing multi-phase modulator circuits.
Conventional digital modulators employ very high-order filters to remove abrupt phase transitions and equalizer circuits to obtain linear-band phase response.